The present invention relates generally to the art of welding power supplies. More specifically, it relates to welding power supplies and the control thereof for short circuit welding.
There are many types of welding power supplies and welding processes. One welding process is referred to as short circuit transfer welding. Short circuit transfer welding generally consists of alternating between an arc state and a short circuit, non-arc state. During the arc state the wire melts, and during the short circuit state the metal further melts and the molten metal is transferred from the end of the wire to the weld puddle. The metal transferred in one cycle is referred to herein as a drop, regardless of the size or shape of the portion of metal that is transferred.
Short circuit transfer welding has many advantages, such as shorter arc length and less melting of the base plate. However, short circuit transfer welding has disadvantages, such as increased spatter.
Both the power source topology and the control scheme must be considered when designing a short circuit transfer welding power source. The power topology used must be fast enough to have a timely response to the chosen control scheme. The control should address three considerations: First, arc length must be properly controlled. Second, the burn-off (or mass deposition) rate must be appropriately controlled. Inappropriate burn-off rate will result in increased spatter. Third, spatter is also caused by too much power when the short is cleared, i.e., the transition from a short circuit to an arc. Thus, the power or current when the short clears must also be controlled. Also, when the short is about to clear must be detected. Some prior art patents do not teach control of the short circuit transfer welding process on a short circuit by short circuit basis. Such a control will provide more precise control of the welding process and will help to reduce spatter.
One common prior art power source topology uses secondary switchers to control the output. While these may provide fast control, they may be relatively expensive or have insufficient peak current capacity. Also, switching high current may increase reliability problems and switching losses. Examples of patents that have secondary switchers include: U.S. Pat. No. 4,469,933, entitled Consumable Electrode Type Arc Welding Power Source, issued Sep. 4, 1984; U.S. Pat. No. 4,485,293, entitled Short Circuit Transfer Arc Welding Machine, issued Nov. 27, 1984; U.S. Pat. No. 4,544,826 entitled Method and Device For Controlling Welding Power Supply to Avoid Spattering of the Weld Material, issued Oct. 1, 1985; U.S. Pat. No. 4,717,807, entitled Method and Device For Controlling a Short Circuiting Type Welding System, issued Jan. 5, 1988.
The control scheme in many prior art power supplies uses arc voltage to determine if arc length is proper. Typically, if the arc voltage is less than a setpoint, the arc length is determined to be too short, and if the arc voltage is greater than the setpoint, arc length is determined to be too long. The output current is controlled to either increase or decrease the amount of metal being transferred, thus controlling the arc length. Some prior art short circuit transfer welding patents taught control of the mass deposition (burn-off) rate by controlling the welding power by xe2x80x9ctotalizingxe2x80x9d the energy delivered to the arc. Arc or welding power is a function of arc current and arc voltage.
However, the burn-off rate on a short-by-short basis (i.e. for any given short circuit transfer welding cycle) is largely independent of arc voltagexe2x80x94it is predominantly a function of arc current. Thus, prior art control schemes that use arc power (or arc energy) to control the burn-off rate are complex, and inaccurate. Example of such complex and inaccurate control schemes include: U.S. Pat. No. 4,866,247, entitled Apparatus and Method of Short Circuiting Arc Welding, issued on Sep. 12, 1989; U.S. Pat. No. 4,897,523, entitled Apparatus and Method of Short Circuiting Arc Welding, issued on Jan. 30, 1990; U.S. Pat. No. 4,954,691, entitled Method and Device For Controlling A Short Circuit Type Welding System, issued on Sep. 4, 1990; and U.S. Pat. No. 5,003,154 entitled Apparatus and Method of Short Circuiting Arc Welding, issued on Mar. 26, 1991. Some of these prior art patents teach control of the power when a short is clearing by predicting the clearing of the short. They generally compare arc voltage or its first derivative to a threshold. However, the prior art attempts result in missed or false positive short clearing predictions.
Accordingly, a short circuit transfer welding power supply that adequately controls the burn-off rate, preferably on a short-by-short basis, is desired. Preferably, the process should be controlled such that power is reduced when the short is clearing. Also, the power source used should be sufficiently fast to respond to the control, but not unduly expensive or limited in peak output current.
One of the causes of instability in a short circuit transfer welding process relates to excessive pre-heating of the wire. Variations in the wire/puddle interaction caused by operator movement and/or changing puddle geometry, can result in irregular pre-heating of the wire due to I2*R heat generation. Too much pre-heating of the wire can cause the melting rate of the wire to increase to a point where the molten ball grows very quickly following the transition from a short to an arc. This quick melting, known as a flare-up, results in a rapid increase in arc length with a corresponding voltage increase.
The opposite extreme can also occur. If there is insufficient pre-heating of the wire, the short circuit frequency will increase as subsequent arc times become shorter. If energy is not added quickly enough, the wire can eventually xe2x80x9cstubxe2x80x9d into the puddle. The end result of such stubbing is either an explosive short clearing, or a sustained short circuit with no arc (sometimes called noodle welding). Over and under preheating often occur in a cyclic fashion. Unfortunately, most prior art controls adjust after a stub or flare-up has occurred. For example, when the control causes the heat to decrease to compensate for past pre-heating, the process has already cycled to the under-heating stage. Thus, the control actually exacerbates the problem. Accordingly, it is desirable to have a short circuit transfer welding process that accurately compensates for the pre-heating of the wire.
It is desirable to have consistent arc starting in most welding processes. The size of the ball at the end of the wire (formed when the last weld was terminated) is a significant factor in determining the amount of energy needed to initiate the arc. Thus, the condition of the end of the wire (size of the ball) from the previous weld should be consistent to provide consistent arc starting.
However, the size of the ball can vary from 1 to 3 times the diameter of the wire after a typical short circuit transfer welding process has ended. Previously, sometimes an operator cut the end of the wire, which eliminated the ball, or in some prior robotic arc spray systems an extra step to dress or trim the wire at the end of each weld and to insure the wire isn""t frozen to the welded joint at arc end is provided (U.S. Pat. No. 5,412,175 issued May 2, 1995, e.g.). While this may produce a uniform wire diameter at the start of the next weld, it wastes time, and the extra step would not be needed if the wire had a consistent diameter when each weld is stopped.
There have been attempts in the prior art to control the termination of a welding process. A BETA-MIG(copyright) has used a predetermined xe2x80x9ccraterxe2x80x9d for the stops. However, the BETA-MIG(copyright) did not provide a fast enough response, or an adequate control scheme, to produce the consistent ball size desired for short circuit transfer welding.
Another prior art system is in the Miller 60M(copyright) pulsed spray process, which has an algorithm that reduces the output pulse frequency to match the stopping of the motor. A final pulse is sent which blows one last ball off the wire and extinguishes the arc. However, this method will not work for processes such as short circuit transfer welding, that do not tightly control the frequency of the output power. Also this prior art does not desirably compensate for irregularities in the process, such as unintended shorts.
Accordingly, a power source and controller that provide a stop algorithm that reduces the size of the ball to be about that of the wire diameter, or of a size that allows consistent starts to be made, i.e. not a large ball, when the process is terminated, is desirable. This process will, preferably, insure that the wire is not frozen to the weld joint at arc end. Also, the stop algorithm should preferably be robust (i.e. able to function even during irregularities in the process) and adaptable for a variety of processes, such as MIG processes, spray processes, pulsed spray processes, or short circuit transfer processes.
According to a first aspect of the invention, a welding process and apparatus includes depositing drops of molten metal at the end of a welding wire into a weld puddle. A power source has a current output in electrical communication with the welding wire. A feedback circuit provides a real-time signal indicative of the heat input to each drop. A controller is coupled to the power source and has a feedback input coupled to the feedback circuit. It controls the magnitude of the current provided to the welding wire in response to the heat of each drop.
One aspect of the invention is that the feedback includes a current signal representative of the output, and the controller determines the power delivered to the wire. The controller also determines when the short is about to clear in response to the power delivered. The controller may determine a rate of change of the output power.
Another aspect is that the controller determines a value Vc defined by Vc=k*(dP/dt), where Vc is a calculated value, k is a scalar, and dP/dt is the derivative of the power. The controller compares Vc to a threshold. The controller subtracts a value responsive to the rate of change of the output current from the rate of change of the output power, in another embodiment.
The controller takes the derivative of a value responsive to the rate of change of the output power less the value responsive to the rate of change of the output current, in another embodiment. Also, the controller determines a value Vc defined by Vc=d/dt(k1*dP/dtxe2x88x92k2*di/dt), wherein k1 is a scalar, dP/dt is the derivative of the output power, k2 is a scalar, and di/dt is the derivative of the output current.
The controller provides a desired mass deposition rate responsive to a wire feed speed and a distance from a tip of the wire to the workpiece, in another alternative.
The controller compares a value responsive to the energy needed to deposit a given amount of wire to a value representing the amount of energy delivered in at least a portion of one welding cycle, in another embodiment. The controller determines the energy needed in accordance with Qreq=k3 (Rdep * (Hm+(Tdropxe2x88x92Tamb) * Cp) * ttot), where Qreq is the energy needed, k3 is a scalar, Rdep is a wire mass deposition rate, Hm is a latent heat of melting for the wire, Tdrop is the temperature of the molten drop, Tamb is the ambient temperature of the wire, Cp is the heat capacity of the wire, and ttot is a cycle length. The controller determines the energy delivered in accordance with Qwire=((Vanode+WF+3 kT/2 e) * I+I2*l*rho/A), where Qwire is the energy delivered, Vanode is the anode voltage drop, WF is the work function of the metal comprising the wire, (3 kT/2 e) is the thermal energy of electrons impinging on the wire, I is the output current, l is the contact tip to arc distance, rho is the resistivity of the wire, and A is the cross sectional area of the wire.
The controller determines a length of stick out (i.e., the length of the wire that extends from the contact tip), in another embodiment. Stick out is determined by providing an arc voltage setpoint, and comparing the arc voltage setpoint to the arc voltage. Then the comparison is integrated over time. The integrand is summed with an integrated burn rate error, and the sum is compared to known values.
Another embodiment includes stopping the welding process. The status of the arc is monitored, and the current is increased in response to the forming of a short circuit. Then, the current is driven to a low current level when the short has cleared, such that a large ball at the end of-the wire is not formed. This is repeated until a short does not occur and the wire stops.
Another embodiment provides that the wire feed speed is monitored, and the stopping of the process begins when the wire feed speed drops below a threshold. In various embodiments the welding process is a MIG, spray, pulse spray, globular or short circuit transfer welding process. In other embodiments the arc is monitored by monitoring the arc voltage.
Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description and the appended claims.